The goal of the present proposal is to further characterize three specific chloride transport processes that exist in barnacle giant muscle fibers and squid giant axons. These are a Cl/Cl exchange process in barnacle muscle, an apparent Cl/HCO3 (or Cl/NaCO3) exchange process in muscle and nerve which is involved in the regulation of intracellular pH, and a Na-Cl co-transport mechanism in the squid axon. Techniques to be used include internal dialysis, internal perfusion and ion-selective microelectrodes.